Among the various video display systems available in the art, an optical projection system is known to be capable of providing high quality displays in a large scale. In such an optical projection system, light from a lamp is uniformly illuminated onto an array of, e.g., M.times.N, actuated mirrors, wherein each of the mirrors is coupled with each of the actuators. The actuators may be made of an electrodisplacive material such as a piezoelectric or an electrostrictive material which deforms in response to an electric field applied thereto.
The reflected light beam from each of the mirrors is incident upon an aperture of, e.g., an optical baffle. By applying an electric signal to each of the actuators, the relative position of each of the mirrors to the incident light beam is altered, thereby causing a deviation in the optical path of the reflected beam from each of the mirrors. As the optical path of each of the reflected beams is varied, the amount of light reflected from each of the mirrors which passes through the aperture is changed, thereby modulating the intensity of the beam. The modulated beams through the aperture are transmitted onto a projection screen via an appropriate optical device such as a projection lens, to thereby display an image thereon.
In FIGS. 1A to 1G, there are illustrated manufacturing steps involved in manufacturing an array 100 of M.times.N thin film actuated mirrors 101, wherein M and N are integers, disclosed in a copending commonly owned application, U.S. Ser. No. 08/430,628, now U.S. Pat. No. 5,636,070 entitled "THIN FILM ACTUATED MIRROR ARRAY".
The process for manufacturing the array 100 begins with the preparation of an active matrix 10 comprising a substrate 12, an array of M.times.N transistors (not shown) and an array of M.times.N connecting terminals 14.
In a subsequent step, there is formed on top of the active matrix 10 a thin film sacrificial layer 24 by using a sputtering or an evaporation method if the thin film sacrificial layer 24 is made of a metal, a chemical vapor deposition (CVD) or a spin coating method if the thin film sacrificial layer 24 is made of a phosphor-silicate glass (PSG), or a CVD method if the thin film sacrificial layer 24 is made of a poly-Si.
Thereafter, there is formed a supporting layer 20 including an array of M.times.N supporting members 22 surrounded by the thin film sacrificial layer 24, wherein the supporting layer 20 is formed by: creating an array of M.times.N empty slots (not shown) on the thin film sacrificial layer 24 by using a photolithography method, each of the empty slots being located around the connecting terminals 14; and forming a supporting member 22 in each of the empty slots located around the connecting terminals 14 by using a sputtering or a CVD method, as shown in FIG. 1A. The supporting members 22 are made of an insulating material.
In a following step, an elastic layer 30 made of the same insulating material as the supporting members 22 is formed on top of the supporting layer 20 by using a Sol-Gel, a sputtering or a CVD method.
Subsequently, a conduit 26 made of a metal is formed in each of the supporting members 22 by: first creating an array of M.times.N holes (not shown), each of the holes extending from top of the elastic layer 30 to top of the connecting terminals 14, by using an etching method; and filling therein with the metal to thereby form the conduit 26, as shown in FIG. 1B.
In a next step, a second thin film layer 40 made of an electrically conducting material is formed on top of the elastic layer 30 including the conduits 26 by using a sputtering method. The second thin film layer 40 is electrically connected to the transistors through the conduits 26 formed in the supporting members 22.
Then, a thin film electrodisplacive layer 50 made of a piezoelectric: material, e.g., lead zirconium titanate (PZT), is formed on top of the second thin film layer 40 by using a sputtering method, a CVD method or a Sol-Gel method, as shown in FIG. 1C.
In an ensuing step, the thin film electrodisplacive layer 50, the second thin film layer 40 and the elastic layer 30 are patterned into an array of M.times.N thin film electrodisplacive members 55, an array of M.times.N second thin film electrodes 45 and an array of M.times.N elastic members 35 by using a photolithography or a laser trimming method until the thin film sacrificial layer 24 in the supporting layer 20 is exposed, as shown in FIG. 1D. Each of the second thin film electrodes 45 is electrically connected to a corresponding transistor through the conduit 26 formed in each of the supporting members 22 and functions as a signal electrode in the thin film actuated mirrors 101.
Next, each of the thin film electrodisplacive members 55 is heat treated at a high temperature, e.g., for PZT, around 650.degree. C., to allow a phase transition to take place to thereby form an array of M.times.N heat treated structures (not shown). Since each of the heat treated thin film electrodisplacive members 55 is sufficiently thin, there is no need to pole it in case it is made of a piezoelectric material: for it can be poled with the electric signal applied during the operation of the thin film actuated mirrors 101.
After the above step, an array of M.times.N first thin film electrodes 65 made of an electrically conducting and light reflecting material is formed on top of the thin film electrodisplacive members 55 in the array of M.times.N heat treated structures by first forming a layer 60, made of the electrically conducting and light reflecting material, completely covering top of the array of M.times.N heat treated structures, including the exposed thin film sacrificial layer 24 in the supporting layer 20, using a sputtering method, as shown in FIG. 1E, and then selectively removing the layer 60, using an etching method, resulting in an array 110 of M.times.N actuated mirror structures 111, wherein each of the actuated mirror structures 111 includes a top surface and four side surfaces, as shown in FIG. 1F. Each of the first thin film electrodes 65, functions as a mirror as well as a bias electrode in the thin film actuated mirrors 101.
The preceeding step is then followed by completely covering the top surface and the four side surfaces in each of the actuated mirror structures 111 with a thin film protection layer (not shown).
The thin film sacrificial layer 24 in the supporting layer 20 is then removed by using an etching method. Finally, the thin film protection layer is removed to thereby form the array 100 of M.times.N thin film actuated mirrors 101, as shown in FIG. 1G.
There are certain deficiencies associated with the array 100 of M.times.N thin film actuated mirrors 101 thus manufactured. The first and foremost is an adhesivity between the thin films constituting each of the actuated mirrors 101. When each of thin film actuated mirror 101 deforms in response to an electric field applied across the thin film electrodisplacive member 55, the first and second thin film electrodes 65, 45 incorporated therein also deform. In the thin film actuated mirrors 101, there is a likelihood of the first thin film electrode 65, the second thin film electrode 45 and the thin film electrodisplacive member 55 delaminating from each other after a prolonged use due to the lack of similarity in the material properties constituting the first and the second thin film electrodes 65 and 45, which are made of a metal, and the thin film electrodisplacive member 55, which is usually made of an electroceramic, e.g., PZT.
Furthermore, a top surface of the thin film electrodisplacive member 55 coming in contact with the first thin film electrode 65 is thermodynamically unstable, and when it is subjected to a high temperature process, islands are formed on a top surface of the thin film electrodisplacive member 55. When the first thin film electrode 65 which also acts as the mirror is formed on such a surface, the resulting mirror surface is not completely flat, affecting the optical efficiency of the thin film actuated mirror 101.